1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus which obtains a magnetic resonance signal generated by applying a gradient magnetic field and a radio frequency pulse into an object in a static magnetic field and reconstructs an image of the object using the obtained magnetic resonance signal, and a radio frequency coil unit. More particularly, this invention relates to a magnetic resonance imaging apparatus including a plurality of surface coils for receiving a magnetic resonance signal and a radio frequency coil unit.
2. Description of the Related Art
According to a related art, a magnetic resonance imaging apparatus is used as a monitoring apparatus in the medical field. The magnetic resonance imaging apparatus forms gradient magnetic fields in X axis, Y axis and Z axis directions using a gradient magnetic field coil in an imaging area of the object disposed in a tubular magnet forming a static magnetic field and transmits a RF (radio frequency) signal from a RF (radio frequency) coil to resonate a nuclear spin in an object. The magnetic resonance imaging apparatus further reconstructs a MR image of the object using an excited NMR signal.
In the magnetic resonance imaging apparatus as described above, surface coils are disposed in a plurality of target regions of the object in order to receive a NMR signal, which makes it possible to rapidly take an image as compared with a technique according to the related art referred to as parallel imaging.
FIG. 15 is a diagram showing an example in which a plurality of surface coils are disposed around an abdominal region of the object in parallel imaging by a magnetic resonance imaging apparatus according to a related art.
As shown in FIG. 15, when imaging the whole abdominal region of the object P, generally, a plurality of surface coils 1 are arranged to surround the object P. A magnetic resonance imaging apparatus is configured to obtain a NMR signal from the whole abdominal region of the object by the surface coils 1. Therefore, the magnetic resonance imaging apparatus can obtain an image for every imaging area with good sensitivity by appropriately arranging the plurality of surface coils for each of the imaging areas. FIG. 15 is a view showing an example that receives reception signals from six surface coils 1 through six receiving channels 2.
According to the parallel imaging method, it is possible to reduce a data acquisition time to 1/n (n is a number of coils arranged in an encode direction when acquiring the maximum number of image data). When imaging an axial section of a body of FIG. 15, if the encode direction is x-direction, three surface coils are arranged. Therefore, the data acquisition time can be approximately reduced to ⅓.
The parallel imaging method is further developed in recent years, and high speed imaging can be performed with reduced error. Therefore, the number of surface coils or the number of receiving channels of a system tends to be increased. For example, a system in which four to thirty-two channels are provided even for a magnetic resonance imaging apparatus having a smaller number of surface coils is produced.
However, there is a problem in that since it is required to dispose the surface coils for every imaging area, the number of surface coils is increased. Further, whenever the object or the imaging area is changed, the user needs to change the surface coils correspondingly to the imaging area. Therefore, the user needs to sufficiently prepare surface coils dedicated to imaging areas. However, changing the surface coil is troublesome for a doctor or an operator.
Generally, in the magnetic resonance imaging apparatus, it is required that the number of receiving channels corresponds to the maximum number of the coils that are used in the imaging area. Therefore, even when the number of surface coils is large, if the number of receiving channels of the magnetic resonance imaging apparatus is small, the number of surface coils that are to be simultaneously used is limited. As a result, there is a problem in that the surface coils can not receive the signals from a larger region the body of the object.
A magnetic resonance imaging apparatus in which a switch circuit or a combining circuit (Matrix) is provided for a plurality of surface coils arranged in a X-axis direction vertical to a body axis of the object to select a mode of a combination of surface coils used for receiving a signal has been suggested (for example, JP-A-2003-334177). In the magnetic resonance imaging apparatus having the switch circuit or the combining circuit, signals received from a proper number of surface coils are combined by selecting a mode of the corresponding surface coil, which makes it possible to perform parallel imaging with a number of channels smaller than that of the surface coils. In JP-A-2003-334177, a switch circuit that is capable of outputting reception signals received by maximum eight surface coils as at least one reception signal is suggested.
FIG. 16 is a conceptual diagram showing an example configured to combine reception signals received by a plurality of surface coils using a combining/switch circuit in parallel imaging by a magnetic resonance imaging apparatus according to a related art.
As shown in FIG. 16, when six surface coils 1 are arranged around the abdominal region of an object P, two combining circuits 3 are provided. Therefore, when the combining circuit 3 combines the signals from three surface coils 1 to one signal and then outputs it, it is possible to receive the reception signals from the six surface coils 1 through two receiving channels 2.
It is further possible to combine various types of reception signals from the respective surface coils and select a mode of reception signals by providing a distributing/combining circuit (for example, refer to “Mode Matrix—A Generalized Signal Combiner For Parallel Imaging Arrays” A. Reykowski, M. Blasche, 2004 Proceedings On CD-ROM, International Society for Magnetic Resonance in Medicine, Twelfth scientific meeting, Kyoto, Japan 15-21 May 2004, pp 1587).
FIG. 17 is a diagram showing a sensitivity distribution of three target surface coils among six surface coils arranged around the abdominal region of the object in parallel imaging by a magnetic resonance imaging apparatus according to a related art. FIG. 18 is a circuit diagram of the distributing/combining circuit for combining signals received from the target surface coils in the magnetic resonance imaging apparatus according to a related art, as shown in FIG. 17.
It is possible to set a plurality of modes in order to combine signals received from the three surface coils L, M, R having the sensitivity distribution 4 as shown in FIG. 17. For example, as shown in FIG. 18, the distributing/combining circuit 5 is configured by a 0°-180° hybrid circuit 6 and a 0°-90° hybrid circuit 7. A signal obtained by combining two input signals is output from a 0° output side of the 0°-180° hybrid circuit 6, and a composite signal obtained by combining two signals whose phases are shifted by 180° is output from a 180° output side of the 0°-180° hybrid circuit 6. Further, a signal obtained by combining two input signals is output from a 0° output side of the 0°-90° hybrid circuit 7, and a composite signal obtained by combining two signals whose phases are shifted by 90° is output from a 90° output side of the 0°-90° hybrid circuit 7.
More specifically, the 0°-180° hybrid circuit 6 is provided at output sides of the surface coils L and R, and the 0°-90° hybrid circuit 7 is provided at the 180° output side of the 0°-180° hybrid circuit 6 and an output side of the surface coil M. A signal output from a channel at the 0° output side of the 0°-180° hybrid circuit 6 is denoted by a signal B, a signal output from a channel at the 0° output side of the 0°-90° hybrid circuit 7 is denoted by a signal C, and a signal output from a channel at the 90° output side of the 0°-90° hybrid circuit 7 is denoted by a signal A.
With this configuration, the signal B is a signal that combines (adds) a reception signal from the surface coil L and a reception signal from the surface coil R. Further, the signal A is a QD signal corresponding to a reception signal from a QD (quadrature detection) surface coil arranged by overlapping a figure-of-eight-shaped surface coil consisting of the surface coils L and R and the surface coil M as a loop surface coil. The signal C is an anti-QD signal that is reversed with the QD signal.
It is known that the QD surface coil can improve SNR (signal to noise ratio) by shifting the phases of the reception signal from the loop surface coil and the reception signal from the figure-of-eight-shaped surface coil by 90° and adding them. In this case, in order to combine the signal from the surface coil M corresponding to the loop surface coil and composite signal of the surface coils L and M that are shifted 180° and forms the figure-of-eight-shaped coil, the 0°-90° hybrid circuit 7 is used.
FIG. 19 is a diagram showing the sensitivity distribution 4 formed by the surface coils L, M, R when the output signal A from the distributing/combining circuit 5 shown in FIG. 18 according to a related art is selected as a reception signal, FIG. 20 is a diagram showing the sensitivity distribution 4 formed by the surface coils L, M, R when the output signal C from the distributing/combining circuit 5 shown in FIG. 18 according to a related art is selected as a reception signal, and FIG. 21 is a diagram showing the sensitivity distribution 4 formed by the surface coils L, M, R when the output signal B from the distributing/combining circuit 5 shown in FIG. 18 according to a related art is selected as a reception signal.
As shown in FIGS. 19, 20, and 21, when performing mode selection by combining the reception signals from the surface coils L, M, R, it is possible to form a plurality of different sensitivity regions 4 by the combination of the same surface coils L, M, R without changing the surface coils L, M, R. Therefore, it is further possible to obtain three image patterns that are reconfigured by the reception signals. Furthermore, it is possible to select a reception signal that is used for imaging in accordance to the number of reception channels included a receiving system of the magnetic resonance imaging apparatus.
For example, as shown in FIG. 17, when six surface coils 1 are provided, a distributing/combining circuit same as the distributing/combining circuit 5 shown in FIG. 18 is provided for three surface coils 1 that face the three surface coils L, M, R. Therefore, when output signals corresponding to the signals A, B, C are denoted as signals A′, B′, C′, respectively, and the number of receiving channels provided in the receiving system is two, it is possible to select the signals A and A′ as the reception signals. Since the QD signal has a wide sensitivity region, it is possible to image a wider area even when the number of the reception channel is only two.
Further, when the number of the receiving channels provided in the receiving system is four, the signals A, B, A′, and B′ can be selected as the reception signals to be used for imaging. When the number of the receiving channels provided in the receiving system is six, the signals A, B, C, A′, B′, C′ can be selected as the reception signals to be used for imaging. Therefore, it is possible to perform high speed imaging corresponding to the number of receiving channels.
Recently, a magnetic resonance imaging apparatus includes a switch circuit that selects or combines reception signals from the surface coils so as to perform parallel imaging even when the number of receiving channels, M and the number of surface coils, N satisfies the relationship of N≧M.
However, in the magnetic resonance imaging apparatus according to the related art, if the number of surface coils increases, when the switch circuit performs both selection and combination of the reception signals, there are problems such as the complicated arrangement of the switch circuit, increased size of the circuit board, increased power consumption, and difficulty in quality insurance.
Further, in recent years, in order to image a wider range, a magnetic resonance imaging apparatus in which a plurality of surface coils are arranged not only in XY direction vertical to a body axis of the object, but also in the body-axis direction is suggested. In the magnetic resonance imaging apparatus having the above mentioned structure, when the number of receiving channels in the body-axis direction is small, it is difficult to cover signals from a wide region of the body of the object regardless of the plurality of surface coils. On the other hand, when the switch circuit selects a reception signal in the body-axis direction, the circuit becomes complicated due to the increased number of the surface coils.